The influence of host physiology on type C retrovirus expression in the normal and tumor-bearing host is being examined. Activation of retrovirus information during pregnancy and by estrogens in reproductive tissues of mice has been demonstrated by several methods including molecular hybridization, electron microscopy, radioimmunoassay and RNA-directed DNA polymerase assay. Further, a component has been extracted from the placenta, and to a lesser extent from the gravid uterus, that exhibits antiviral activity which has biophysical, biochemical and immunological characteristics of virus induced (Type 1) interferon. These results are consistent with our hypothesis that retroviruses participate in normal physiologic processes.